Fiber optics technology has advanced to such a level that it is now possible to fabricate a product having a transmission loss reduced essentially to the theoretical limit. However, most of the long fibers that can be used for communications without repeaters have transmission losses greater than the theoretically possible level, and this is due largely to the uneven distribution of refractive index in an axial direction. Optical fibers are generally produced by a method comprising hydrolyzing (oxidizing) a gaseous glass forming material with the flame from an oxyhydrogen burner, and depositing the resulting soot in a rod shape which is then sintered in an electric furnace to make transparent glass which then is drawn to form a fiber. The soot usually contains a dopant that modifies the refractive index of the glass, so if the gas current in the protective vessel (muffle furnace) wherein the soot deposits to form the porous optical fiber preform becomes turbulent, the flame flickers, causing uneven distribution of refractive index.
The conventional apparatus for producing an optical fiber preform is so designed that a given amount of a gas such as an inert gas (e.g., helium) or air is supplied to the muffle furnace continuously. The gas is not involved in the flame hydrolysis at all, but removes the waste gases from the deposition such as unreacted silicon tetrachloride gas, as well as hydrogen chloride gas and steam formed as by-products from the muffle furnace as quickly as possible so that impurities do not deposit on the rod of the optical fiber preform once formed.
However, the present inventors have found that when a gas at room temperature is supplied to the muffle furnace having relatively high temperatures, violent convection currents form in the muffle furnace, causing the flame to flicker greatly. The surface temperature of the porous optical fiber being formed in the muffle furnace, especially that part which is at the tip of the supporting rod that faces the flame, has a great effect on the rate at which the soot is formed and deposited. The present inventors have also found that variations in the temperature of the gas supplied to the muffle furnace eventually causes a change in the surface temperature of the face of the soot.